In a conventional plasma welding and cutting tool or gun, an operating gas is ionized by an arc formed between a nozzle and a electrode of different electrodes to form plasma for welding and cutting work pieces. Since there is not provided with any device in a welding or cutting gun to keep a constant operating pressure for the operating gas from a gas inlet towards a discharge orifice at the nozzle, the fluctuation of inlet gas pressure may influence the plasma velocity and temperature through the nozzle orifice to cause temperature fluctuations of the plasma towards the work piece, thereby being unable to perform the welding and cutting operation at constant operating temperature, resulting in poor quality and efficiency of the welding and cutting job, and resulting in unhomogeneous stress or strength of finished work piece under unstable operating condition deteriorate the reliability and durability of the engineering or product by the conventional welding and cutting gun.